wire/breaker size for transformer

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Kenny G

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Can someone help me understand how to calculate wire and breaker size for a 3phase 225kva transformer. we have a 120/208y connection at this facility and are getting a new machine that draws 465 amps at 460 volts, so the company is providing a 225kva step up transformer. I need to know what size wire and breaker i need to run this bad boy.... any help would be great !
Thank You in advance !
 
Agree with iwire,to small,,,taking it one step further 465 amps x 125%=581,a 500kva xfmr is about right at 601amps/480


dick
 
Sounds too small to me!

Sounds too small to me!

:confused:

I agree with Bob. 460V x 465a= 213,900 VA x 1.73 (square root of 3 for 3-phase calculations) = 370047VA or 370KW. Your transformer seems undersized for the loading, unless I am truly missing something here. Sounds like your vendor is selling you short on power supply. You also need to take into account transformer loss (efficiency rating), the type you are purchasing (energy star compliant, etc.) The cheaper the transformer you are buying, the more you will pay in power usage over the years. Better upfront payout for copper windings vs. aluminum, higher quality insulation (in the transformer core) .... all of these and more things will yield lower operational costs in the long-run.

Is this machine rated at 465a continuous (3 hours or more) or is it a machine that runs for a short time and is cycled on/off as needed?


Ken Sellars - Intrepid
 
Intrepid said:
Better upfront payout for copper windings vs. aluminum, higher quality insulation (in the transformer core) .... all of these and more things will yield lower operational costs in the long-run.
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I can not let it pass (there is no intention for a thread hijack). Copper vs aluminum is not important unless you hold other design criteria fixed. If I have full control, I can design a very efficient aluminum wound transformer and a very inefficient copper unit. Each material has advantages and disadvantages, one is not inherently worse than the other.


Kenny,

What else do you know about the transformer output? Is it going to be a delta or wye connected unit. have you considered the grounding issues of the 480V side? What is the load profile? Some times designers will use the overload capacity of a transformer to handle short time loads because they know the normal operating profile is substantially less (for example, bringing an cold furnace on line takes more power than maintaining the temperature of a hot one).
 
Thank you for the replies.....

Thank you for the replies.....

yes you are correct the amp draw is 255amps, I have spec sheet on this machine if I can attatch or post it...... No i dont have any other info on the transformer they havent even give me the brand name.
 
Kenny G said:
yes you are correct the amp draw is 255amps, I have spec sheet on this machine if I can attatch or post it...... No i dont have any other info on the transformer they havent even give me the brand name.
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What is important is how you are going to handle the grounding on the 480V side of the transformer.

In all cases you need and equipment grounding conductor/system.

A delta wired unit can be left ungrounded, but then you need a ground detection/indicator system or, a delta can be corner grounded, but then you need protective devices that are rated for corner grounding (typically a problem with breakers).

A wye unit can be solidly grounded, just like any 480Y/277 system or, it can be high resistance grounded if there are no L-N loads.

In cases like this, I often treat single machines differently than I do distributed loads (ie. a panelboard).
 
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